Capacitive microphone with integrated cavity

ABSTRACT

A capacitive microphone and method of fabricating the same are provided. One or more holes can be formed in a first printed circuit board (PCB). A diaphragm can be surface micro-machined onto an interior surface of the first PCB at a region having the one or more holes. Interface electronics can also be interconnected to the interior surface of the PCB. One or more spacer PCBs can be attached to a second PCB to the first PCB, such that appropriate interconnections between interconnect vias are made. The second PCB and first PCB with spacers in between can be attached so as to create a cavity in which the diaphragm and interface electronics are located.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.12/597,572, filed Dec. 1, 2009, now U.S. Pat. No. 8,705,775, which isthe U.S. National Stage Application of International Patent ApplicationNo. PCT/US2008/061603, filed Apr. 25, 2008, which claims the benefit ofU.S. Application Ser. No. 60/926,307, filed Apr. 25, 2007, all of whichare hereby incorporated by reference herein in their entirety, includingany figures, tables, or drawings.

BACKGROUND OF INVENTION

Many consumer electronic products incorporate a microphone. As this is ahigh volume market, average selling price (ASP) is typically a keyfactor. The “Yole Silicon Microphone Market Report 2005” [1] projectsthe total silicon microphone market to be 2.75 M units and 221 M$ by2008 with the major applications being mobile phones, PDAs, laptops,PCs, hearing aids, acoustic noise control and automotive crashdetection. Many of these products use conventional electret condensermicrophones (ECM) produced by a number of low cost suppliers. To date,one silicon micromachined microphone (Knowles acoustics SiSonic [2]) hasbeen able to compete effectively in this market by meeting theperformance, reliability, and price expectations set by the ECMsuppliers. Typically, die shrinkage (more parts per wafer) andelimination of processing steps (lower cost per fabrication lot run) canlower the ASP. However, the high costs associated with siliconmicrofabrication currently limits the microfabrication-based costcutting measures. The packaging costs of the device can also be adominant factor. For the SiSonic microphone, the packaging structureincludes a base, a wall, and a lid all made from FR4 printed-circuitboard (PCB) material and laminated together [2]. This package must belarge enough to fit the silicon microphone and amplifier die, as well asthe associated passives.

BRIEF SUMMARY OF INVENTION

Embodiments of the subject invention relate to a method of fabricating acapacitive microphone. Embodiments also pertain to a capacitivemicrophone. In an embodiment, the subject capacitive microphone can usePCB-fabrication technology to realize a low-cost microphone integratedwith the microphone package. FIG. 1 shows a cross-section of a specificembodiment of a capacitive microphone in accordance with the subjectinvention. The embodiment of FIG. 1 is a condenser microphone in which asurface micromachined diaphragm 10 is separated from a porous backplate11 on the interior surface of a sheet of a PCB substrate 12. Thediaphragm 10 can be positioned on the PCB substrate 12 such that a frontsurface of the diaphragm can be exposed to air passing through one ormore apertures 13 formed in the PCB substrate 12. The microphone in FIG.1 can also be an electret microphone by placing permanent electriccharge on either the back plate or the diaphragm and creating an outputsignal based on a change in voltage across the back plate and thediaphragm.

Embodiments of the subject capacitive microphone can be condenser orelectret condenser. In a specific embodiment fabricated using PCB-basedtechnology, a 24″×24″ substrate is utilized, which can save costs forhigh volume. In an embodiment locating the interface electronics withinthe cavity, a conductive interior surface on the enclosure top, wall, orbottom can be used to connect the backplate to the interfaceelectronics, and connecting to the diaphragm, such that no wire bondsare needed. Reducing the need for wire bonds can reduce costs andimprove reliability. In an embodiment, the exterior of the package canbe metal-plated and grounded to shield against electromagneticinterference. In accordance with various embodiments of the invention,lower fabrication cost and an integrated package can allow themicrophone diaphragm to be much larger. The larger diaphragm can improvesensitivity, increase the sensor capacitance, and reduce the noisefloor, resulting in superior performance. Embodiments of the inventioncan incorporate a large back volume such that the microphone can reducecavity stiffening effects with respect to silicon devices that arelimited to a silicon wafer thickness resulting in improved deviceperformance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional schematic of a PCB-based capacitivemicrophone according to an embodiment of the present invention.

FIG. 2 shows a cross-sectional schematic of a PCB-based capacitivemicrophone according to an embodiment of the present invention.

FIG. 3 shows a cross-sectional representation of a PCB board accordingto an embodiment of the present invention.

FIGS. 4A and 4B show a cross-sectional view and top view, respectively,of drilled holes fabricated according to an embodiment of the presentinvention.

FIGS. 5A-5D show cross-sectional schematics illustrating a method offabricating a microphone according to an embodiment of the presentinvention.

FIGS. 6A-6C show cross-sectional schematics illustrating a method offabrication a microphone according to an embodiment of the presentinvention.

DETAILED DISCLOSURE

Embodiments of the subject invention relate to a method of fabricating acapacitive microphone. Embodiments also pertain to a capacitivemicrophone. In an embodiment, the subject capacitive microphone can usePCB-fabrication technology to realize a low-cost microphone integratedwith the microphone package. FIG. 1 shows a cross-section of a specificembodiment of a capacitive microphone in accordance with the subjectinvention. The embodiment of FIG. 1 is a condenser microphone in which asurface micromachined diaphragm 10 is separated from a porous backplate11 on the interior surface of a sheet of a PCB substrate 12. Thediaphragm 10 can be positioned on the PCB substrate 12 such that a frontsurface of the diaphragm, in contact with portion 23 a of cavity 16, canbe exposed to air passing through one or more apertures 13 formed in thePCB substrate 12 such that portion 23 a is in low acoustic impedancewith the ambient acoustic environment outside of the device. Themicrophone in FIG. 1 can also be an electret microphone by placingpermanent electric charge on either the back plate or the diaphragm andcreating an output signal based on a change in voltage across the backplate and the diaphragm.

Referring to FIG. 1, the two terminals 14 a and 14 b of the condenserare attached to the appropriate surface mounted interface electronics 15(i.e., charge pump, buffer amplifier, etc.) via PCB leads. The interfaceelectronics 15 can be located interior to the PCB package microphone. Ina specific embodiment, a capillary vent hole can be created through thepackage, e.g., through the top or sides, to allow air to flow in and outof the portion of the cavity 16 in contact with the surface of diaphragm10 opposite the surface of the diaphragm 10 toward the one or moreapertures 13, in order to equilibrate pressure within the cavity 16. Thevent allows portion 23 b of cavity 16 to be in high acoustic impedancecontact with the ambient acoustic environment outside of the device. Inan alternative embodiment, venting can be accomplished via apertures inregion 17 or, for embodiments utilizing an insulating spacer between theconductive material connected to the backplate 11 and the diaphragm 10,the apertures can be in the insulating spacer. Such venting can beprovided so as to allow a meaningful signal response below about 10 Hzby maintaining portion 23 b of cavity 16 in high acoustic impedancecontact with the ambient acoustic environment. A middle PCB layer 18 canbe hollowed out to form a spacer layer between the top PCB layer 12 andbottom PCB layer 19, such that a cavity 16 is formed. The PCB layers 18and 19 can include standard conductive vias 20 to the backside of thebottom PCB 19 and appropriate solder bumps and/or leads 21 to enablebump-bond/re-flow assembly.

An optional dust cover 22 can be provided over the one or more apertures13. The dust cover 22 can be in the form of a protective mesh. In aspecific embodiment, the dust cover 22 can be a felt top.

FIG. 2 shows a cross-section of another specific embodiment of acapacitive microphone in accordance with the subject invention. Theembodiment of FIG. 2 is a condenser microphone in which a diaphragm 30is separated from a backplate 31 on the interior surface of a sheet of aPCB substrate 32 such that a back surface of the diaphragm 30, incontact with portion 40 a of cavity 37, can be exposed to air passingthrough one or more apertures 33 formed in another PCB substrate 34,such that portion 40 a of cavity 37 is in low acoustic impedance withthe ambient acoustic environment outside of the device. A front surfaceof the diaphragm 30 is in contact with portion 40 b of cavity 37 betweendiaphragm 30 and back plate 31, which is in high acoustic impedance withthe ambient acoustic environment. Interface electronics 35 can bemounted on the PCB substrate 32. The interface electronics 35 can belocated interior to the PCB package microphone. In one embodiment, amiddle PCB layer 36 can be hollowed out to form a spacer layer betweenthe top PCB layer 34 and bottom PCB layer 32, such that a cavity 37 isformed. The PCB layer 32 can include standard conductive vias 38 to thebackside of the bottom PCB 32. In a further embodiment, an optional dustcover 39 can be provided over the one or more apertures 33. Themicrophone of FIG. 2 can also be an electret microphone by placingpermanent electric charge on either the back plate or the diaphragm andcreating an output signal based on a change in voltage across the backplate and the diaphragm.

Embodiments of the subject capacitive microphone can be condenser orelectret condenser. In a specific embodiment fabricated using PCB-basedtechnology, a 24″×24″ substrate is utilized, which can save costs forhigh volume. In an embodiment locating the interface electronics withinthe cavity, a conductive interior surface on the enclosure top, wall, orbottom can be used to connect the backplate to the interfaceelectronics, and connecting to the diaphragm, such that no wire bondsare needed. Reducing the need for wire bonds can reduce costs andimprove reliability. In an embodiment, the exterior of the package canbe metal-plated and grounded to shield against electromagneticinterference. In accordance with various embodiments of the invention,lower fabrication cost and an integrated package can allow themicrophone diaphragm to be much larger. The larger diaphragm can improvesensitivity, increase the sensor capacitance, and reduce the noisefloor, resulting in superior performance. Embodiments of the inventioncan incorporate a large back volume such that the microphone can reducecavity stiffening effects with respect to silicon devices that arelimited to a silicon wafer thickness resulting in improved deviceperformance.

In a condenser embodiment of the subject microphone, the microphone canwithstand higher operating temperatures and can withstand lead-freesolder re-flow cycles (e.g., around 400° C.), which is a productassembly advantage over ECMs. The enclosure of various embodiments ofthe invention can use a variety of materials, including as examplesprinted circuit board (PCB) or printed wiring board (PWB). PCB and PWBtechnology refer to modern circuit board construction. These boards caninclude multiple laminated dielectric and conductive layers. Thedielectric layer can serve as the structural support. FR4(flame-retardant 4) can be used as the dielectric layer in the boards.Other options include, but are not limited to, FR2, polyimide (forflexible circuits), Getek, Thermount, and Rogers 4050, Rogers 4003 (RFcircuits), etc. The conductive layers (e.g., copper or other metal) canbe etched or “patterned” to provide discrete electrical connectionsbetween various regions of the board.

In an embodiment, surface-micromachining can be used to form amicrophone directly on the board substrate forming a portion of theenclosure. Referring to FIG. 3, a first step can involve selecting a PCB200 having an outer conductor layer 201 on one or both exterior surfacesand one or more dielectric layers 202 in the middle. The dielectriclayer(s) 202 can be fiber glass and the conductor layer 201 can be ametal. Referring to FIGS. 4A and 4B, where FIG. 4A shows across-sectional view and FIG. 4B shows a top view, one or more apertures203 can be created through the PCB board 200 to allow air to travelthrough the PCB. In one embodiment, the one or more apertures 203 can beformed by drilling into the PCB 200. Other techniques for creating theapertures can also be used. In a variety of embodiments, the position,shape and pattern of the one or more apertures 203 can be formed asdesired.

In one embodiment, patterns can be etched in the outer conductorlayer(s) 201 of the PCB board 200 in preparation of interconnecting theelectronics of the microphone. FIG. 5A shows a specific embodiment of aPCB 200A having patterns etched into the conductor layer at one surfaceof the PCB 200A. The patterns can include diaphragm lead lines 201A, abackplate pattern 201B, and interface electronics lead lines 201C.

Referring to FIG. 5B, a diaphragm 204 can then be surface micromachinedonto the interior surface of the PCB 200A. In addition, the interfaceelectronics 205, which can be provided in the form of an ApplicationSpecific Integrated Circuit (ASIC), can be interconnected to theinterior surface of the PCB 200A either before or after micromachiningthe diaphragm 204 onto the PCB surface. Referring to FIG. 5C, one ormore spacer PCBs 206 can be attached to a bottom PCB 207, or to the topPCB 200A, such that appropriate interconnections between interconnectvias 208 are made. Referring to FIG. 5D, the bottom PCB 207 and top PCB200 with spacers 206 in between can be attached so as to create a cavity209 in which the diaphragm 204 and ASIC interface electronics 205 arepositioned. It is understood that other techniques can be used to createthe cavity.

In another embodiment as illustrated in FIGS. 6A-6C, a diaphragm can belocated on a second PCB that is not provided with one or more apertures.Referring to FIG. 6A, patterns can be etched in the outer conductorlayer(s) of a PCB substrate 207A in preparation of interconnecting theelectronics of the microphone. The patterns can include diaphragm leadlines 201D, a backplate pattern 201E, and interface electronics leadlines 201F. Interconnect vias can be formed in the PCB substrate 207A toprovide electrical connections to the outside.

Referring to FIG. 6B, a diaphragm 204 can then be surface micromachinedonto the interior surface of the PCB 207A. In addition, the interfaceelectronics 205, which can be provided in the form of an ApplicationSpecific Integrated Circuit (ASIC), can be interconnected to theinterior surface of the PCB 207A either before or after micromachiningthe diaphragm 204 onto the PCB surface. Referring to FIG. 6C, one ormore spacer PCBs 206 can be attached to the PCB 207A or to a top PCB 200having one or more apertures 203. Then, the bottom PCB 207A and top PCB200 with spacers 206 in between can be attached so as to create a cavity209 in which the diaphragm 204 and ASIC interface electronics 205 arepositioned. It is understood that other techniques can also be used tocreate the cavity.

When constructing a complete package with the ASIC interface electronicsand, optionally other components embedded, the system can be amulti-chip module (MCM), where MCM technology refers to assembling oneor more devices, chips, or components on a common substrate to form amore complex system. MCMs can be further classified by the supportingtechnology used to form the electrical interconnections on thesubstrate. Embodiments of the invention can utilize MCM-L, MCM-D, and/orMCM-C, where MCM-L (laminated MCM) involves a base substrate that is amulti-layer laminated PCB, MCM-D (deposited MCM) involves a basesubstrate that is often a semiconductor wafer with films deposited usingthin film deposition techniques, and MCM-C (ceramic substrate MCM)involves a base substrate that is laminated ceramic board, (e.g.low-temperature co-fired ceramic (LTCC)) most often used for RFcircuits.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

REFERENCES

[1] “SIMM '05, Yole Silicon Microphone Market Report 2005 Technology andMarket Analysis: From Silicon Microphone Device to Microphone Modules”,Yole Development, September 2005.

[2] P. V. Loeppert and S. B. Lee, “SiSonic™—The first commercializedMEMS microphone,” in Proceedings of Solid-State Sensor and ActuatorWorkshop, Hilton Head Island, S.C., 2006, pp. 27-30.

The invention claimed is:
 1. A microphone, comprising: a back platefabricated on a portion of a printed circuit board; and a diaphragmpositioned relative to the portion of the printed circuit board so as toform a first volume between the diaphragm and the back plate, wherein afront surface of the diaphragm is in contact with the first volume,wherein a back surface of the diaphragm is in contact with a secondvolume, wherein one of the first volume and the second volume is in lowacoustic impedance contact with the ambient acoustic environment and theother of the first volume and the second volume is in high acousticalimpedance contact with the ambient acoustic environment, wherein whenthe diaphragm flexes in response to acoustic pressure, an output signalrelated to the acoustic pressure on the diaphragm is produced; and anenclosure, wherein the first volume is within the enclosure; wherein thefirst volume is in high acoustic impedance contact with the ambientacoustic environment the second volume is in low acoustic impedancecontact with the ambient acoustic environment; and wherein the enclosurecomprises: the printed circuit board; a bottom printed circuit board;one or more spacer printed circuit boards, wherein the printed circuitboard, the one or more spacer printed circuit boards, and the bottomprinted circuit board are attached to form the enclosure, wherein thediaphragm is within the enclosure.
 2. The microphone according to claim1, wherein the diaphragm comprises a conducting material, wherein theback plate comprises a conducting material, wherein when a bias voltageis applied between the diaphragm and the back plate, flexing of thediaphragm causes a change in electric charge across the diaphragm andthe back plate, wherein the output signal is produced from the change inelectric charge across the diaphragm and the back plate.
 3. Themicrophone according to claim 2, further comprising a means for applyinga bias voltage between the back plate and the diaphragm.
 4. Themicrophone according to claim 3, wherein the means for applying a biasvoltage between the back plate and the diaphragm comprises interfaceelectronics.
 5. The microphone according to claim 3, further comprisingan electrical impedance buffer amplifier that produces the output signalrelated to the acoustic pressure on the diaphragm.
 6. The microphoneaccording to claim 5, wherein the output signal related to the acousticpressure on the diaphragm is a low impedance output signal.
 7. Themicrophone according to claim 1, wherein the diaphragm comprises apermanent electric charge, wherein the back plate comprises a conductingmaterial, wherein flexing of the diaphragm causes a voltage changeacross the diaphragm and the back plate, wherein the output signal isproduced from the voltage change across the diaphragm and the backplate.
 8. The microphone according to claim 1, wherein the diaphragmcomprises a conducting material, wherein the back plate comprises apermanent electric charge, wherein flexing of the diaphragm causes avoltage change across the diaphragm and the back plate, wherein theoutput signal is produced from the voltage change across the diaphragmand the back plate.
 9. The microphone according to claim 1, wherein thefirst volume is formed by the diaphragm and the back plate.
 10. Themicrophone according to claim 1, further comprising one or moreapertures through the printed circuit board that allow acoustic waves topass through the one or more apertures from the ambient acousticenvironment into the second volume and from the second volume to theambient acoustic environment.
 11. The microphone according to claim 1,wherein the diaphragm is micromachined on the printed circuit board. 12.The microphone according to claim 11, wherein the diaphragm is grown ordeposited onto the printed circuit board.
 13. The microphone accordingto claim 1, wherein the diaphragm is placed on the printed circuitboard.
 14. The microphone according to claim 1, further comprising avent passing through the enclosure that allows ambient fluid to passback and forth between the first volume and the ambient acousticenvironment while maintaining the first volume in high acousticimpedance with ambient acoustic environment.
 15. The microphoneaccording to claim 14, wherein the ambient fluid is air.
 16. Themicrophone according to claim 1, further comprising: a vent to allowambient fluid to pass back and forth between the second volume and thefirst volume while maintaining the first volume in high acousticimpedance with the ambient acoustic environment.
 17. The microphoneaccording to claim 16, wherein the ambient fluid is air.
 18. Amicrophone, comprising: a back plate deposited on a portion of a firstprinted circuit board; a diaphragm positioned relative to the portion ofthe first printed circuit board so as to form a first volume between thediaphragm and the back plate, wherein a front surface of the diaphragmis in contact with the first volume, wherein a back surface of thediaphragm is in contact with a second volume, wherein one of the firstvolume and the second volume is in low acoustic impedance contact withthe ambient acoustic environment and the other of the first volume andthe second volume is in high acoustical impedance contact with theambient acoustic environment, wherein when the diaphragm flexes inresponse to acoustic pressure, an output signal related to the acousticpressure on the diaphragm is produced; and an enclosure, wherein thefirst volume is within the enclosure; wherein the first volume is inhigh acoustic impedance contact with the ambient acoustic environmentthe second volume is in low acoustic impedance contact with the ambientacoustic environment; wherein the enclosure comprises: the first printedcircuit board; a second printed circuit board; one or more spacerprinted circuit boards, wherein the first printed circuit board, the oneor more spacer printed circuit boards, and the second printed circuitboard are attached to form the enclosure, wherein the diaphragm iswithin the enclosure.
 19. The microphone according to claim 18, whereinthe diaphragm comprises a conducting material, wherein the back platecomprises a conducting material, wherein when a bias voltage is appliedbetween the diaphragm and the back plate, flexing of the diaphragmcauses a change in electric charge across the diaphragm and the backplate, wherein the output signal is produced from the change in electriccharge across the diaphragm and the back plate.
 20. The microphoneaccording to claim 19, further comprising a means for applying a biasvoltage between the back plate and the diaphragm.
 21. The microphoneaccording to claim 20, wherein the means for applying a bias voltagebetween the back plate and the diaphragm comprises interfaceelectronics.
 22. The microphone according to claim 20, furthercomprising an electrical impedance buffer amplifier that produces theoutput signal related to the acoustic pressure on the diaphragm.
 23. Themicrophone according to claim 22, wherein the output signal related tothe acoustic pressure on the diaphragm is a low impedance output signal.24. The microphone according to claim 18, wherein the diaphragmcomprises a permanent electric charge, wherein the back plate comprisesa conducting material, wherein flexing of the diaphragm causes a voltagechange across the diaphragm and the back plate, wherein the outputsignal is produced from the voltage change across the diaphragm and theback plate.
 25. The microphone according to claim 18, wherein thediaphragm comprises a conducting material, wherein the back platecomprises a permanent electric charge, wherein flexing of the diaphragmcauses a voltage change across the diaphragm and the back plate, whereinthe output signal is produced from the voltage change across thediaphragm and the back plate.
 26. The microphone according to claim 18,wherein the first volume is formed by the diaphragm and the back plate.27. The microphone according to claim 18, further comprising one or moreapertures through the printed circuit board that allow acoustic waves topass through the one or more apertures from the ambient acousticenvironment into the second volume and from the second volume to theambient acoustic environment.
 28. The microphone according to claim 18,wherein the diaphragm is micromachined on the printed circuit board. 29.The microphone according to claim 28, wherein the diaphragm is grown ordeposited onto the printed circuit board.
 30. The microphone accordingto claim 18, wherein the diaphragm is placed on the printed circuitboard.
 31. The microphone according to claim 18, further comprising avent passing through the enclosure that allows ambient fluid to passback and forth between the first volume and the ambient acousticenvironment while maintaining the first volume in high acousticimpedance with ambient acoustic environment.
 32. The microphoneaccording to claim 31, wherein the ambient fluid is air.
 33. Themicrophone according to claim 18, further comprising: a vent to allowambient fluid to pass back and forth between the second volume and thefirst volume while maintaining the first volume in high acousticimpedance with the ambient acoustic environment.
 34. The microphoneaccording to claim 33, wherein the ambient fluid is air.